Synergistic Structural Regulation of Starch-Derived Hard Carbon via NH4H2PO4-Mediated Pre-oxidation for Enhanced Sodium Storage

Spherical starch-derived hard carbon (HC) is a promising anode material for sodium-ion batteries owing to its high packing density (afforded by its spherical morphology) and exceptional thermal stability. However, carbonization often causes starch particles to foam, resulting in the loss of their spherical morphology. Additionally, conventional air pre-oxidation is time-consuming. Therefore, developing an efficient air-stabilization strategy for producing spherical HC with superior Na+ storage performance remains a critical challenge. Herein, a structural modulation strategy employing NH4H2PO4-catalyzed pre-oxidation is proposed for the production of starch-derived HC. NH4H2PO4 catalyzes oxidative cross-linking to develop a stable network containing P-O-C and C=O bonds, which effectively suppresses foaming during carbonization and preserves the spherical morphology of the starch-derived HC. Furthermore, NH4H2PO4 facilitates the formation of narrow and elongated graphitic microcrystals, thereby generating abundant closed pores during carbonization. When tested at 0.02 A g-1, the resultant HC exhibits a reversible capacity of 365.7 mAh g-1. These findings provide a viable structural design strategy for fabricating biomass-derived HC.